Enhancing retarding potential analyzer energy measurements with micro-aligned electrodes

Abstract

Plasmas are ionized gases, and constitute a large fraction of the known universe. For example, solar wind is a plasma that emanates from the sun reaching the Earth's magnetosphere. At times these ionized species cause beautiful auroral displays over the planet's magnetic poles. Moreover, when a hypersonic object enters the atmosphere, the shock wave that is generated induces a plasma sheath that surrounds the object. The resulting plasma is hot and dense and may cause material ablation from the surface of the object. Other plasmas of similar or greater density exist in fusion reactors, and in silicon processing chambers. A Retarding Potential Analyzer (RPA) is a sensor that measures the ion energy distribution of a plasma. The ion energy influences the ablation of surfaces, or plasma etching, as well as the deposition processes. Integrated circuit foundries could greatly benefit from a diagnostic tool such as an RPA in plasma chambers. Measuring particle energy during reactive ion etching, ion implantation, ion milling, plasma enhanced chemical vapor deposition, etc, in situ would close the control loop to improve the uniformity and repeatability of numerous processes. In order to measure the ion energy of a plasma, an RPA utilizes a system of grids with holes smaller than a few Debye length - a characteristic length proportional to the square root of the electron temperature divided by the electron number density. Thus, cold, dense plasmas have the smallest associated Debye lengths and require smaller grid openings than can be achieved using conventional machining. In this thesis an improved RPA design is proposed that utilizes the following three key concepts: (i) the aperture size and inter-electrode spacing required by dense plasmas are defined using micro electromechanical systems (MEMS) processing techniques; (ii) aperture alignment across successive grids is mechanically enforced to improve the transmission of ion species through the device; (iii) densely packing apertures in each RPA grid multiplexes the signal onto the collector. A MEMS RPA is built with apertures as small as 100 pm in diameter having an inter-grid spacing of only 200 pm. These are the narrowest aperture and gap dimensions for an RPA with enforced electrode alignment to date. The new RPA design is benchmarked against the present state of the art downstream of an ion source from a mass spectrometry (MS) system. An ion source is chosen because of the fine control it offers over the ion energies, as a low energy with little variability increases mass resolution in MS systems. Through enforced alignment, the MEMS RPA shows an order of magnitude increase in signal strength over a conventional RPA. In improving the transmission of ions through the sensor, the artificial broadening of RPA ion energy distribution measurements is mitigated, resulting in a threefold improvement in sensor energy resolution. This is characterized by a reduction in the full width half maximum (FWHM) value from 2.5 V for the conventional device down to 0.85 V for the MEMS RPA. The various RPAs are then tested in a helicon plasma, capable of replicating many dense plasmas in the range of 1 x 10¹⁶ m-³ to 1 x 10¹⁸ m-³. Langmuir probe measurements provide estimates of the electron temperature and plasma density, from which the Debye length is derived. In these experiments, only the new RPA designs were able to effectively trap the plasma down to a Debye length of approximately 50 [mu]m and obtain ion energy distributions. The range of application for RPAs is thus expanded through the use of microfabrication techniques.